1. Field of the Invention
The present invention relates generally to a plastic container, and more particularly to a plastic container having a base structure that enhances the structural integrity of the container. The present invention also relates to a preform for forming a plastic container having a base structure that enhances the structural integrity of the container and a method for hot filling a plastic container with a product.
2. Related Art
Plastic containers are commonly used to package a wide variety of liquid, viscous or solid products including, for example, juices, other beverages, yoghurt, sauces, pudding, lotions, soaps in liquid or gel form, and candy. Such containers can be made by conventional blow molding processes including, for example, extrusion blow molding, stretch blow molding, and injection blow molding. A plastic container can generally be filled with any contents intended to be contained therein and can then be sealed or capped to form a sealed enclosure.
Many conventional containers are configured and formed to withstand the rigors of so-called hot fill processing. In a hot fill process, a liquid product is added to the container at an elevated temperature which can be near the glass transition temperature of the plastic material, and the container is then capped. As the container and its contents cool, the contents tend to contract and this volumetric change creates a partial vacuum within the container. In the absence of some means for accommodating these internal volumetric and barometric changes, containers tend to deform and/or collapse. For example, a round container can undergo ovalization, or tend to distort and become out of round. Containers of other shapes can become similarly distorted. In addition to these changes that adversely affect the appearance of the container, distortion or deformation can create weak portions in the container walls. Such deformation can also cause the container to become unstable, particularly when distortion of the base region occurs.
One well known arrangement for overcoming or withstanding these tendencies includes simply adding more material to the outside structural walls of the container. This solution, however, can be costly, not only in terms of the additional material required for each container, but also in terms of shipping and handling of mass quantities of heavy containers. End consumers are also generally more amenable to lighter-weight containers in terms of ease of use and waste product reduction. Thus, lightweight plastic containers that still meet particular strength requirements are more desirable to both product manufacturers and consumers alike.
Another known solution is the introduction of hinged vacuum panels on a portion of the container. Hinged panels are generally employed in hot filled plastic containers to effectively absorb volumetric changes created by the partial vacuum within the container upon cooling. Although this arrangement allows lightweight plastic containers to overcome the volumetric changes resulting from hot fill processing while still maintaining overall strength and shape, the hinged vacuum panels may not provide a desired aesthetic appearance such as, for example, the look of a smooth glass bottle.
More recently, in order to avoid the need for providing the hinged vacuum panels in a portion of a hot filled container, it has been proposed to offset the vacuum effects associated with hot filling by introducing a liquefied gas such as, for example, liquid nitrogen, into the container prior to capping. Specifically, once the container is hot filled with the contents, a liquefied gas injection system introduces a predetermined amount of the liquefied gas into the hot filled container and the container is then sealed and/or capped. Thereafter, the liquefied gas undergoes a phase change from liquid form to gas form, thereby increasing the positive internal pressure of the container. The positive internal pressure created within the container is a function of the inherent properties of the particular liquefied gas utilized as well as the amount injected, the temperature of the hot filled material, and the time between injection of the liquefied gas and the capping of the container. Some known methods and systems for liquid gas injection are described, for example, in U.S. Pat. No. 5,251,424 to Zenger et al., U.S. Pat. No. 6,182,715 B1 to Ziegler et al., and U.S. Patent Application Publication No. 2005/0011580 A1 to Ziegler et al., all of which are hereby incorporated by reference in their entirety.
One particular problem that arises in lightweight containers that are hot filled and injected with liquefied gas, however, is eversion, or so-called “rollout.” For example, when the liquefied gas is injected into the container and the container is then capped, the positive internal pressure created by the phase change of the liquified gas can tend to cause at least some portion of the container to evert, or bulge, outwardly (i.e., “rollout”). This not only presents a problem in terms of overall aesthetic appearance of the container, but also in terms of the practical and functional aspects of the container, such as when such rollout occurs in the base of the container. In this respect, the container may no longer be able to stand upright, thus ultimately affecting stacking, shipping, and overall consumer end use of the container.
What is needed, therefore, is an improved plastic container base structure that provides the necessary structural integrity to prevent eversion or rollout of the base portion when a positive internal pressure arises within the container.